The electrodes in gas-discharge lamps such as those used for digital projection lighting (DPL) become very hot during operation of the lamp. In particular, operating conditions in ultra high-pressure (UHP) gas-discharge lamps are such that temperatures of 1200 K are easily reached in the coolest area of the lamp, namely the pinch area. The temperature at the tips of the electrodes can easily reach 3700 K. At such high temperatures, the thermal load on an electrode is extreme, and the tip of the electrode can melt back and significantly alter the shape of the electrode. This is known as electrode burn-back. When both electrodes are shortened by burn-back, the separation between the front faces of the electrodes lengthens, as does the discharge arc, so that the luminance of the discharge arc is lessened.
The “light source” is the discharge arc in the case of an arc-discharge lamp, and the size and shape of the light source is directly related to the electrode separation. A small electrode separation is generally favourable since this delivers a near point-size light source with a small etendue. The etendue of the light source should match the etendue of the optical system for an optimal projector or “beamer” performance. For example, an optical panel can be based on an array of micro-mirrors on a semiconductor chip in a digital micromirror device. Since the cost of an optical panel is related to its size, the relatively large electrode separation of prior art electrodes in UHP lamps is also a cost factor in the manufacture of optical panels for projection systems using those lamps. The evolution towards smaller optical panels makes a smaller electrode separation desirable, so that the effects of burn-back exhibited by prior art electrodes can be a serious drawback.
One way of improving the thermal behaviour of an electrode that is subject to an increased thermal load would be to increase its mechanical stability, so that it would be less prone to burn-back during operation. For example, large solid electrodes could be used. However, such large electrodes are correspondingly heavy and would require a complete re-design, including adjustments to driving scheme parameters of a lamp driver.
In another known approach, a coil of tungsten wire is arranged on the electrode shaft. Usually, the coil is formed by winding wire in one or more layers around a ‘dummy needle’ and then transferring the completed coil onto the electrode shaft. During operation, the coil acts as a good thermal radiator and can serve to obtain a better balance between the input and output power of the electrode. However, even for such a coil-and-rod electrode, the unavoidably high temperature at the tip of the electrode will melt the electrode tip. Therefore, the shape of the prior-art coil-and-rod electrode will alter significantly so that the lamp behaviour changes during the first operating hours until a stable electrode surface is obtained. Therefore, some manufacturing methods for a coil-and-rod electrode include a step in which the altered stable operation shape of the electrode is obtained in advance, for example by melting the tip of the electrode and some of the coil to form a fused area at the front face of the electrode. A method to do this is by laser-melting the electrode tip.
The known coil-and-rod electrode designs are associated with a number of disadvantages. High power DPL lamps suffer from a specific type of electrode failure, since it may happen that parts of the coil ‘open’ or even break during lamp operation as a result of the high thermal load. While coil breakage occurs quickly and effectively terminates the lamp lifetime, coil opening can significantly shorten the lamp lifetime, so that both of these negative developments are highly undesirable. Furthermore, coil ‘opening’ means that the coil unwinds slowly under the thermal load, with a corresponding negative effect on the electrode's thermal characteristics. For example, an electrode with an ‘opened’ coil may be associated with an increase of lamp operating voltage, since the coil no longer fulfils its function and the electrode is subject to a greater thermal load. Also, the high thermal load in the electrode results in the very undesirable burn-back of the electrode front faces.
Therefore, it is an object of the invention to provide an improved electrode design for a gas-discharge lamp.